Companies are finding it difficult to use renewable energy to cut their carbon emissions, despite targets set by central government and local authorities, the conference heard.

The government aims to more than triple renewable energy to 15% of the energy mix by 2015, but companies, which account for some 40% of greenhouse gas emissions, are struggling to do their bit.

The Co-operative Group is one of the boldest in investing in renewable technologies in the UK: a 7,000-panel solar array that clads the CIS insurance building in Manchester is Europe's largest vertical installation, and the roof of another CIS building is carpeted with micro-wind turbines.

Paul Monaghan, head of sustainability at the Co-op, said the company will also have 100GW of electricity coming from wind turbines on its farms by the end of this year.

"We believe in micro generation as the future," he said. "But it isn't working. You have to be an altruist to put in on your buildings. There's no way they work economically." But he said unless companies like the Co-op invest in micro generation, the market will never become big enough to drive down prices.

It was because it wanted to help boost demand that the London borough of Merton brought in a planning requirement that all major developments need to have 10% of their energy needs met by onsite energy generation. The so-called Merton Rule has been adopted in some form by many councils across the country, and, with typical boldness, adopted - and then doubled - by London mayor Ken Livingstone.

Adrian Hewitt, climate change team leader at Merton, believes the bottom up pressure for the UK to introduce German-style feed-in tariffs to encourage renewable energy production "will become irresistible". In Germany, 83% of residential electricity and 87% of business electricity have renewables in the mix.

One big problem here, though, is a mismatch between supply and demand. There is already three times more corporate demand for renewable energy in the UK this year than the electricity generated in 2006. And nearly five times the current capacity for wind energy is being held up in the planning system, according to the Co-op.

Francis Sullivan, environment adviser at carbon-neutral HSBC, said his company sources 30% of its energy needs worldwide from renewables. "The country we have the most difficulty buying green electricity from is the UK," he said.

HSBC offsets 50% of its CO2 emissions, but the bank wants to bring this down. Although it is far cheaper to offset than to invest in renewables or in energy efficiency, Sullivan says, the latter are a better investment bet.

He said in an interview: "Although you have to spend more [on energy efficiency and renewables], they give you permanent savings. With offsets you keep spending in perpetuity."

He added that companies that do not find it difficult to brave the green electricity market. And supply is not the only problem. With three different regulatory regimes governing green electricity, he said, it is hard to know whether you are getting the real deal. "There's no system that guarantees that the green electricity you are buying is only being sold to you."

M&S has decided to stop buying in green electricity altogether. Instead it wants to turn the farms of its suppliers into renewable energy power plants by investing in anaerobic digestion.

"It's very possible these farms will make more from electricity than the animal husbandry they are involved in," said Richard Gillies, director of store development and procurement at M&S.

Since customers are not prepared to pay more to be green, the company needs to future-proof its renewable energy supply, he said. "We're doing this in a sustainable way that doesn't put our shareholders or customers at risk."